1. Field of the Invention
This invention relates to a contact charging device for charging a member to be charged such as an image bearing member mounted in a copying apparatus, a laser beam printer or the like, and an image forming apparatus.
2. Related Background Art
FIG. 4 of the accompanying drawings is a longitudinal cross-sectional view schematically showing the construction of an image forming apparatus according to the prior art. The construction and operation of the image forming apparatus will hereinafter be described briefly with reference to FIG. 4.
An original G is first placed on an original supporting table 11 with the image surface thereof (the surface to be copied) turned downward. When copying is then started, the image surface of the original G is scanned while being irradiated from below it by a unit 12 comprised of an original irradiating lamp, a short-focus lens array, a CCD sensor, etc. constructed integrally with one another. Thereby, the reflected light from the image surface is imaged by the short-focus lens array and enters the CCD sensor. The CCD sensor is comprised of a light receiving portion, a transfer portion and an output portion. In the light receiving portion, an optical signal is converted into a charge signal, and is sequentially transferred to the output portion by the transfer portion in synchronism with a clock pulse, and in the output portion, the charge signal is converted into a voltage signal, which in turn is amplified and made into low impedance and outputted. An analog signal obtained in this manner is subjected to well-known image processing and is converted into a digital signal, which is sent to a printer portion.
In the printer portion, the above-mentioned image signal is received and an electrostatic latent image is formed in the following manner. A photosensitive drum (image bearing member) 1 is rotatively driven in the direction of arrow R1 at a predetermined peripheral velocity (process speed), and the surface thereof is uniformly charged to about -650 V by a charger 2. An electrostatic latent image is formed on the surface of the photosensitive drum 1 after charged, by a laser scanning portion 100. In the laser scanning portion 100, as shown in FIG. 6 of the accompanying drawings, a solid laser element 102 is first turned on and off at predetermined timing by a light emission signal generator 101 on the basis of the inputted image signal to thereby emit a laser beam. The laser beam emitted from the solid laser element 102 is converted into a substantially parallel light beam by a collimator lens system 103 and is further scanned by a rotatable polygon mirror 104 rotated in the direction of arrow (counter-clockwise direction) and also is imaged on the surface of the photosensitive drum 1 by .function..multidot..theta. lens units 105a, 105b and 105c as indicated by arrow Co in FIG. 6. By the scanning of such a laser beam, an exposure distribution corresponding to one image scan is formed on the surface of the photosensitive drum 1 and further, during each scanning cycle, the surface of the photosensitive drum 1 is scrolled by a predetermined amount in a direction perpendicular to the scanning direction, whereby an electrostatic latent image as an exposure distribution conforming to the image signal is formed on substantially the whole of the surface of the photosensitive drum 1.
The electrostatic latent image is developed by a developing device 3. The developing method is divided broadly into a method of coating a developing sleeve with a non-magnetic toner by a blade or the like, or coating the developing sleeve with a magnetic toner by the magnetic force thereof and conveying the magnetic toner and developing a latent image with the toner in a non-contact state with the photosensitive drum 1 (one-component non-contact development), a method of developing a latent image with the toner coating in the above-described a manner being brought into contact with the photosensitive drum 1 (one-component contact development), a method of using a mixture of toner particles and a magnetic carrier as a developer and conveying the developer by the magnetic force thereof and developing a latent image with the developer in a contact state with the photosensitive drum 1 (two-component contact development), and a method of developing a latent image with the above-mentioned two-component developer in a non-contact state (two-component non-contact development), but the two-component contact developing method is often used from the viewpoints of the high quality and high stability of image.
FIG. 5 of the accompanying drawings schematically show the construction of a developing device for two-component magnetic brush development as an example of the two-component contact developing method. In FIG. 5, the reference numeral 31 designates a developing sleeve for carrying and conveying a developer, the reference numeral 32 denotes a magnet roller fixedly disposed in the developing sleeve 31, the reference numerals 33 and 34 designate agitating screws for agitating the developer, the reference numeral 35 denotes a developing blade for regulating the developer carried on the surface of the developing sleeve 31 into a thin layer, the reference numeral 36 designates a developer container containing the developer therein, and the reference numeral 37 denotes a voltage source for applying a voltage to between the developing sleeve 31 and the photosensitive drum 1. The developing sleeve 31 is disposed relative to the surface of the photosensitive drum 1 so that at least during development, the most proximate area thereof may be about 500 .mu.m, and is set so that development can be accomplished with the developer being in contact with the photosensitive drum 1.
In the two-component developer used in this example of the prior art, titanium oxide of an average particle diameter 20 nm added at 1% by weight to a negatively charged toner of an average particle diameter 6 .mu.m manufactured by the crushing method was used as toner particles, and a magnetic carrier of an average particle diameter 35 .mu.m having saturation magnetization of 205 emu/cm.sup.3 was used as the carrier. Also, this toner and this carrier mixed at a weight ratio 6:94 were used as the developer.
Description will now be made of the developing step of visualizing the above-described electrostatic latent image by the two-component magnetic brush method by the use of the above-described developing device 3, and a circulation system for the developer. First, the developer drawn up by a magnetic pole N.sub.2 with the rotation of the developing sleeve 31 and carried on the surface of the developing sleeve 31 has its layer thickness regulated by the regulating blade 35 disposed perpendicularly to the surface of the developing sleeve 31, in the course of the developer being conveyed from a magnetic pole S.sub.2 to a magnetic pole N.sub.1, and is formed as a thin layer on the developing sleeve 31. The developer thus formed as a thin layer is conveyed to a developing main magnetic pole S.sub.1 and an electromagnetic brush is formed by a magnetic force. This developer formed in the shape of an electromagnetic brush has its toner caused to adhere to the aforedescribed electrostatic latent image and develops the latent image into a toner image, whereafter the toner and carrier not used for the development are collected from the surface of the developing sleeve 31 into the developer container 36 by the repulsive magnetic field of poles N.sub.2 and N.sub.3. The developing sleeve 31 has applied thereto from the voltage source a superposed voltage comprising a DC voltage and an AC voltage superposed one upon the other. In this example of the prior art, -500 V is applied as the DC voltage and a peak-to-peak voltage V.sub.pp =1800 V and a frequency Vf=2000 Hz are applied as the AC voltage. Generally in the two-component developing method, when an AC voltage is applied, the developing effect increases and the quality of image becomes high.
The toner image formed on the photosensitive drum 1 in this manner is electrostatically transferred onto a transfer material such as paper by a transfer charger 4a shown in FIG. 4. After the transfer of the toner image, the transfer material is electrostatically separated from the surface of the photosensitive drum 1 by a separating charger 4b and conveyed to a fixating device 6, where the toner image is heat-fixated and outputted. On the other hand, after the transfer of the toner image, the photosensitive drum 1 has an adhering contaminant such as untransferred toner on its surface removed by a cleaner 5, and has its charges removed by a charge removing lamp and is used for the next image formation.
In the above-described construction, the charger 2 is a corona charger, but contact charging systems (such as magnetic brush charging, fur brush charging and roller charging) have come to be used with a view to suppress the creation of ozone by corona discharging and with a view to achieve a higher quality of image such as being free of wire contamination. Also as to the transfer charger 4a, there are various types including a transfer roller, but basically, as described above, an image is formed by a series of processes such as charging, exposure, development, transfer, fixation and cleaning.
Now, in recent years, the downsizing of image forming apparatuses has progressed, but there has been a limit to the downsizing of the apparatuses by achieving the downsizing of only the instruments for effecting the above-mentioned series of processes such as charging, exposure, development, transfer, fixation and cleaning. The aforementioned untransferred toner is collected by the cleaner 5, but it is preferable from the viewpoint of environmental protection that such waste toner be absent.
So, there has appeared a cleanerless image forming apparatus in which the above-described cleaner 5 is removed and development and cleaning are effected at a time by the developing device 3. Development and cleaning at a time is a method of collecting some untransferred toner remaining on the photosensitive drum after transfer by a defogging potential difference V.sub.back which is the potential difference between the DC voltage applied to the developing device 3 and the surface potential of the photosensitive drum 1 during the development after the next step. According to this method, the untransferred toner is collected and reused after the next step and therefore, waste toner can be eliminated. Also, the absence of the cleaner means an advantage in terms of space, and the downsizing of the entire image forming apparatus becomes possible.
However, when in the above-described image forming apparatus, the cleaner 5 was eliminated and development and cleaning at a time were attempted, the positive ghost of the preceding image was created in a portion free of the image at the period of rotation of the photosensitive drum 1. This positive ghost is a phenomenon occurring with the toner transferred to a portion which originally is a white ground portion because the untransferred toner of the preceding image could not be collected in the developing area. In contrast with this, a contact charging member is used as a charging member and a voltage is applied thereto, whereby it becomes possible to erase the history of the untransferred toner remaining in the state of the preceding image and discharge it onto the photosensitive drum in a uniform state. Therefore, the toner discharged onto the thin layer becomes advantageous in the collectability during development and the fault of image becomes preventable.
However, when outputting was effected by the use of the contact charging member as described above, good images were obtained on several sheets, but when several thousand sheets were supplied, there occurred unsatisfactory charging due to the contamination of the contact charging member by the toner.
Also, the reduction in the charging property due to the contamination of the contact charging member is a problem which causes the formation of bad images not only in the case of the development and cleaning at a time as described above, but also when a special cleaner as shown in FIG. 4 is provided.